1. Field of the Invention
This invention relates to a prestroke adjustment mechanism for a fuel injection pump and more particularly to a prestroke adjustment mechanism for a fuel injection pump capable of reducing the force of internal turbulence owing to spill jetting of fuel.
2. Prior Art Statement
Conventional fuel injection pumps are equipped with a prestroke adjustment mechanism for varying the prestroke. The mechanism generally achieves the adjustment by varying the relative position between a vertically reciprocating plunger and a control sleeve.
Prestroke adjustment mechanisms of this type are taught, for example, by Japanese Utility Model Public Disclosure Nos. Sho 58-114875 and Sho 53-9460.
A typical structural arrangement of the prestroke adjustment mechanism of a fuel injection pump will be explained with reference to FIGS. 3 to 6.
FIGS. 3 to 5 are sectional views of the essential part of a fuel injection pump 1. FIG. 3 shows the pump at the beginning of fuel injection, FIG. 4 shows it at the end of fuel injection, and FIG. 5 shows it at the time of fuel spill.
The fuel injection pump 1 has a pump housing 2, a plunger barrel 3, a plunger 4, a control sleeve 5 and a timing control rod 6.
The pump housing 2 is formed with a fuel inlet 7 and a fuel reservoir 8 extending from the pump housing 2 into the plunger barrel 3, and the plunger barrel 3 is formed with a fuel compression chamber 9. The plunger 4 is reciprocated vertically by rotational driving force from an engine (not shown). As a result, fuel passes from the fuel inlet 7 through the fuel reservoir 8 to the fuel compression chamber 9 where it is pressurized and delivered to an injection nozzle (not shown) through an injection tube 10.
More specifically, the plunger 4 has a fuel suction and discharge hole 11 which constitutes a fuel suction port opening into the fuel reservoir 8, a center communication hole 12 formed axially at its center so as to communicate the fuel suction and discharge hole 11 with the fuel compression chamber 9, an inclined control lead 13 formed on its outer surface, and a vertical groove 14 for communicating the inclined control lead 13 with the orifice of the fuel suction and discharge hole 11.
The control sleeve 5 is fitted on the plunger to be slidable thereon. It is formed with a cutoff hole 15 which passes radially therethrough. The cutoff hole 15 is disposed so to be able to communicate with the inclined control lead 13 in the course of the vertical motion of the plunger 4.
The control sleeve 5 and the timing control rod 6 are linked by an eccentric pin 16. The timing control rod 6 is connected with a rotary solenoid or other such actuator (not shown) and the vertical position of the control sleeve 5 relative to the plunger 4 can be adjusted by rotating the actuator.
The operation of the fuel injection pump 1 of the aforesaid structure will now be explained.
When the plunger 4 first starts to rise from its bottom dead point, the fuel suction and discharge hole 11 is open to the fuel reservoir 8 and, therefore, the fuel reservoir 8 and the fuel compression chamber 9 are in communication via the fuel suction and discharge hole 11 and the center communication hole 12. Because of this, the fuel pressure in the fuel compression chamber 9 does not rise and there is no delivery of pressurized fuel.
As shown in FIG. 3, actual delivery of pressurized fuel starts when the plunger has risen to the point where the fuel suction and discharge hole 11 is closed by the control sleeve 5 so that the fuel pressure in the fuel compression chamber 9 can increase.
The stroke of the plunger 4 between the bottom dead point and the point at which pressurized fuel delivery starts is called the prestroke.
When the plunger 4 rises to the point shown in FIG. 4, the inclined control lead 13 comes into communication with the cutoff hole 15 of the control sleeve 5. As a result, the cutoff hole 15 and the fuel compression chamber 9 are communicated via the inclined control lead 13, vertical groove 14, fuel suction and discharge hole 11 and center communication hole 12 so that the fuel in the fuel compression chamber 9 escapes to the fuel reservoir 8. This is called fuel spill. The pressure of the fuel in the fuel compression chamber 9 therefore decreases and the delivery of pressurized fuel ends.
As can be seen in FIG. 5, the plunger 4 continues to rise even after the completion of pressurized fuel supply so that the inclined control lead 13 rises beyond the cutoff hole 15 to be closed by the control sleeve 5 and the upper part of the inclined control lead 13 projects above the control sleeve 5 into the fuel reservoir 8. As a result, fuel spills from upper part of the inclined control lead 13 into the fuel reservoir 8.
Then when the plunger 4 descends, fuel is sucked from the fuel reservoir 8 into the fuel compression chamber 9 owing to the negative pressure in the fuel compression chamber 9.
As indicated by the arrows in FIG. 3, the timing control rod 6 can be rotated in either direction to raise and lower the control sleeve 5 for varying its position relative to the plunger 4. It is thus possible to adjust the prestroke, i.e., to control the fuel injection timing.
As shown in FIG. 6, however, when the engine is stopped, the force of a spring (not shown) provided in the aforesaid actuator positions the control sleeve 5 to the uppermost position in the fuel reservoir 8, namely the position at which the top 5A of the control sleeve 5 abuts on the bottom 3A of the upper section of the plunger barrel 3. The prestroke is adjusted with the control sleeve in this position for matching the characteristics of the fuel adjustment pump to the requirements of the engine in which it is used.
When a long prestroke is employed with this type of prestroke adjustment mechanism, the gap between the top 5A of the control sleeve 5 and the bottom 3A of the upper section of the plunger barrel 3 becomes narrow. Since the sectional area of the spill jet escape passage therefore becomes small, the pressure of the spill jet becomes large and the high pressure thereof exerts a downward force (force of internal turbulence) on the control sleeve 5. As a result, it becomes impossible to control the prestroke.
The invention was completed in the light of the foregoing problems and has as its object to provide a prestroke adjustment mechanism for a fuel injection pump which enables prestroke adjustment to be achieved in such manner that an adequate gap is established between the top of the control sleeve and the bottom of the upper section of the plunger barrel, thereby preventing adverse effect from the force of internal turbulence and ensuring stable and reliable prestroke control.